Minecraft 1.7.4, Minecraft 14w04b, Minecraft 14w05b, Minecraft 14w06a, Minecraft 14w07a, Minecraft 1.7.9
When lava pools generate in a forest or taiga biome, a nearby tree's leaves can become stone.
How to recreate: Find a taiga or forest biome, and a lava pool. if there is a close enough tree, some of the leaves become stone, granite, andesite or diorite.
this could be fixed if the lava pools were generated after trees, or if leaves were excluded from being replaceable blocks for the pools of lava.
Edit: provided seed, world specs and coords.
superflat specs: 2;7,59x1,3x3,2;5;decoration,lava_lake
World is superflat, its hard to find a default world example. they do exist though.